Natural Gas is used at HALF the pressure (approx. 1/4 psi compared to 1/2 psi with LP) with about TWICE the size oriface as propane. It's due to the specific gravity of the two gasses.
There are tables to figure out the oriface size per BTU required at each burner.
Natural Gas pressure is regulated lower (about 1/2 the pressure of LP) at the regulator in the gas valve. When converting to Propane, the regulator is blocked open to allow the
system supply regulator to supply the required 10" Water Column pressure. Convertible valves have a cap that sets the spring pressure correct for Nat, and flipped over puts more pressure on the spring, actually blocking it open. Valves made for either gas ONLY are not convertible, meaning you can't change the pressure in the regulator. That's the only difference.
Here is a gas oriface sizing chart.
http://www.joppaglass.com/burner/lowp_chrt.html
It requires numbered oriface drills, but it gives the diameter in thousandths as well. I have a set of numbered oriface drills, so never had to measure the one needed with a micrometer, but that's how you can make sure you use the correct size if you don't know the size of the drill bits you have.
Orifaces are normally brass, and can be peened shut with a ball peen hammer (closed) and redrilled from Nat to LP. You will also find them soldered shut and redrilled. The oriface stays cold at the air intake, so this works too. It's nice to have a threaded piece of steel that the oriface screws into used as an ANVIL to protect the threads if you peen it shut. That's why many used solder.
The air adjustment will need to be set on each burner as well to obtain an all blue flame. Yellow tips are a no no and will cause carbon deposits on pans. It's quite simple once you do it. New ranges have adjustible orifaces that come set for Nat. and you simply turn them down tight to close the hole to the LP setting. Some older appliances that were convertible had a set of orifaces screwed to the back to change to the gas being used.
I'm retired from my own LP gas service business and this is what I did, as well as installing systems. There are now laws requiring a lable to be affixed on the appliance and signed by the installer. This identifies which gas the appliance is set up for.